Facial ID will have to be very quick indeed.
My only comparison of a similar system is the iris scanner on the S8. Leaving aside the accuracy, it had to be held in a particular way. The iPhone cannot be like this.
Simple scenario:
Go to pay with my iPhone.
Take iPhone out of my pocket with headphones on.
Phone doesn't go above the waist before reaching the terminal.
Reach terminal with thumb already resting on the home button, acting as support to hold the phone.
Apple Pay active.
Payment done.
How will this work with facial recognition? At some point the phone has to align with my face.
How does this work when the phone is lying down on the table, a few inches away from my face? With Touch ID I simply press the home button.
There are many many questions.